1. Field of the Invention
The present invention relates to a character data writing device and more particularly to a character data writing device used in a calculator.
2. Description of the Related Art
A calculator of liquid crystal display type comprises a CPU (central processing unit), a ROM (read only memory) for storing a program, a key input section, a liquid crystal display section, and a character data writing device.
FIG. 1 shows a conventional character data writing device.
Since this character data writing device is relatively simply structured, the device can be manufactured at a low cost.
Reference numeral 40 is a ROM for storing character data. Reference numeral 41 is a display data switching circuit for selecting character data from the ROM 40 or data from a shift register (display data holding circuit) 42 based on a display data switching command.
A CPU generates a switching command of display data based on the input of the key, which is used in a replacement processing or a calculation processing.
The shift register 42 holds data, which is serially outputted from display data switching circuit 41, and supplies data to display data switching circuit 41.
The shift register 42 supplies data, which is serially outputted from display data switching circuit 41, to display data switching circuit 41, and holds data. The holding operation of display data is continued as long as a switch command of new display data is not generated.
A latch pulse generating circuit 44 supplies a latch pulse to a latch circuit 45 of display data. The latch pulse is outputted every time when a count of a clock pulse, which is inputted to a modulo N counter 43 from the latch pulse generating circuit 44, is ended.
Display data, which is outputted from the shift register 42, is inputted to the latch circuit 45. Display data is synchronized with the latch pulse, and latched by the latch circuit 45. Latched data (for example, bit data of 7 segments) is supplied to each digit place of a crystal display as a segment signal.
In the above-structured character data writing device, timing (writing timing of character data), which is used to latch display data, which is outputted from the shift register 42, to the latch circuit 45, is determined by the latch pulse.
Therefore, the writing of display data is set to be in a standby state by a software processing until the switching command of display data is generated and display data and the latch pulse are synchronized with each other.
Due to this, as shown in FIG. 2, a step of a write waiting command is needed in a part of a routine of the program of the CPU.
However, if the writing of display data is set to be in a standby state by a software processing in displaying character data, there is a disadvantage in that a writing speed of character data becomes low.
On the other hand, it can be considered that a clock frequency is increased so as to make the writing speed of character data higher.
However, many batteries are often used as a power source in the calculator. Therefore, in the calculator, a consumption electrical current, which is as low as possible, is needed in the calculator. In other words, in the calculator, which is presently used, there is a disadvantage in that the clock frequency can not be unnecessarily increased.